Signal-seeking receiver for amplitude modulation and frequency modulation signals



Nov- 1949 M. G. NICHOLSON, JR. ETAL 2,487,772

SIGNAIr-SEEKING RECEIVER FOR AMPLITUDE MODULATION AND FREQUENCY MODULATION SIGNALS Filed Aug. 8. 1945 I INVENTOR 7/746725027 63%23075021 J and WCBWZZZIS ATTORN EY UNITED STATES Patented Nov. 8, 1949 Madison G. Nicholson, Jr., Snyder, and John 0.

PATENT OFFICE.

Pontius, Williamsville, N. Y., assignors' to' Colo nial Radio Corporation, Buffalo, N. Y.

Application August 8, 1945, Serial No. 609,574

9 Claims.

This invention relates to so-called signal-seeking receivers operable both on amplitude modulated and frequency modulated signals.

A signal-seeking receiver, sometimes called stop-on-carrier or signal-tuned receiver, as the term is employed herein, is a receiver in which the incoming signal itself controls the tuning in such a manner that variation of tuning ceases through the action of the receiver itself when a received signal of predetermined strength is properly tuned in.. In the usual form of such receiver, a small electric motor is provided, geared to the tuning instrumentality, such as a variable condenser or variometer, and all circuits required to be variably tuned to select the signal are ganged together and connected to the motor to be driven thereby.

When an incoming signal of predetermined strength is tuned in, the voltage developed operates a relay, usually through the agency of an electronic tube, :to stop the tuning variation, and the receiver will remain tuned to the incoming signal until the operator presses a button to select another station, whereupon th motor begins to drive the tuner and the receiver scans its spectrum until another signal of predetermined strength is tuned in, at which time, as before, the voltage derived from the incoming signal stops the tuning variation.

In other applications filed by Madison. G. Nicholson, Jr., there are described various forms of signal-seeking receivers, some designed to operate on amplitude modulated signals, and others designed to operate on frequency modulated signals; but, as far as we know, no such receiver has ever been designed to operate either on amplitude or frequency modulated signals.

The "problem of making such a receiver to operate on frequency modulatedsigna'ls is much more complicated than to make one operating on amplitude modulated signals. The reason for this is that with amplitude modulated signals, the carrier frequency is constant and, therefore, a selecting circuit tuned to the desired carrier frequency may be utilized and the voltage built up in this applied directly to a control tube;

However, in the case of frequency modulated signals, as is well known, the carrier frequency is not constant, but fluctuates or deviates about a mean value, the deviation rate being in accordance with the audio modulation, and the amount of deviation being proportional :to the intensity of the modulating sounds.

Therefore, the actual carrier itself cannot be utilized to operate the control tube, as in the case. of amplitude modulated receivers, because as the carrier frequency deviates, it would be a mere matter of chanc etwhat point the control tube would operate, an-d since there are many more positions at which the signal. may be heard badly distorted than there arepositions. at which it can be heard without distortion, in most cases the receiver would stop in the wrong position andwould give out a badly' distortedsignal.

In accordance with our invention, we have provided a new and improved receiver which will operate in accordance with the signal-seeking principle, and in which the same controlling apparatus serves to stop the tuningat the proper position where a signal of predetermined strength is being received, whether that signal is amplitude or frequency modulated.

From the foregoing, it will be understood that among the objects of our invention are:

To providea. signal-seeking receiver which will operate in response to'eithertlamplitude or frequency modulatedsignals.

To provide such a; receiver in which the parts required in addition to those required to operate with amplitude modulated signals are very few in number and low in cost.

It is a further object of our invention to provide such a receiver utilizing an automatic frequency control tube maintaining a closer approach to the desired tuning than could otherwise be obtained.

It is still a further object of our invention to provide a relativelysimple, reliable, and inexpensive receiver of the class described.

Still other objects and advantages of-our invention will be apparent from the specification.

The features of novelty which we-believe to be characteristic of our invention are set forth with particularity in the appended claims. Our invention itself, however, both as to i-ts'fundamental principles and as to its particular embodiments, will best be understood by reference to the specification and accompanying drawing, in which The single figure is a-circuit diagram of such parts of the receiver as are necessary to an understanding of the invention.

Referring now more particularly to the drawing, l0 represents an antenna", which may be connected to ground through condenser H and to control grid [593 of a'converter' tube through condenser II. The input circuit may be :tuned in any suitable manner, for example, as herein shown by permeabilitytuning of coil 13 connected between control grid l 5g3wand ground; that is, by variation of the position of commlnuted iron core l4.

Tube 15 maybe of the so-called pentagrid con-' i 'verter type, having cathode E50, grids I5gl, I592, l5g3, i594, and i5g5, and anode lEa. Grids l5g2 and l5g4 may be connected together and to B+. Suppressor i595 may be connected to ground or to cathodel 50 within the tube. Grid 559! may be connected to cathode through resistance 28 and to the oscillator circuit through condenser l9.

Anode i5a may be connected to 13-}- through resistor 10.

The oscillator circuit mayconsist of inductance l6 tuned by iron core I! ganged with iron core l4, and inductance I6 may be closed through condenser l8 and the common point of inductance i6 and condenser 58 connected to ground. An intermediate point on inductance It may be connected to cathode I50.

It will be understood that, whereas the input circuit, tuned by inductance i3, is tuned to the signal, oscillator circuit i6l8 is tuned to a frequency to produce a beat frequency of the desired value by heterodyning the incoming signal. Tube 55 also acts as a first demodulator and supplies beat frequency signals to I. F. amplifier 2 i.

The output of the I. F. amplifier may be supplied to limiter 22, and also to amplitude modulation detector 3! The output from limiter 22 may be supplied through the frequency discriminator network to double diode 26. The frequency discriminator network may be made up of primary coil 23 coupled to secondary coil 24, the primary coil being tuned by variable condenser 25, and secondary coil 24 by condenser 25a, and the high side of condenser 25 may be connected through condenser 26 to the midpoint of inductance 24.

One side of condenser 25a may be connected I to anode 25a! of diode 2t, and the other side of condenser 2511 may be connected to anode 25a2 of diode 26. Cathode 2602 may be connected directly to ground and cathode 260! connected to ground through condenser 51. While we have shown and described diode 25 as a double diode, it will be apparent that two individual diodes may be employed if desired.

Cathode 260! may be connected to ground through resistances 27 and 28 in series, and the common point of these two resistances may be connected to the common point of condensers 25 and inductance 24. Cathode 260! may likewise be connected to switch contact 32, and the output from amplitude modulated detector so may be connected to switch contact 3 l.

Switch arm 33 may be movable to connect either to contact 3! or 32, depending upon whether amplitude modulated or frequency modulated reception is desired, and switch blade 33 may be connected through resistance 34 to ground. A variable point on this resistor may be connected to the input of audio amplifier 35, which supplies suitable loud speaker 36 or other indicator. 7

It Will be understood that audio frequency voltages will appear on contacts 3! and 32, and switch 33 permits the use of the same audio frequency amplifier whether amplitude modulated or frequency modulated signals are being received,,a1-' though it will be understood that two separate audio frequency amplifiers and loud speakers may be provided if the increased expense is not a factor.

Automatic frequency control tube 65 is provided, which may have cathode 600, control grid 64g, screen grid fiflsg, suppressor grid Bilsr, and anode 60a. ground through resistance-6| by-passed by condenser 62. Control grid 609 may be connected Cathode 600 may be connected to '4 through an R. C. circuit consisting of resistances 54 and 55 in series, and condenser 56 connected between resistances 54 and 55 to ground, to cathode lead 260i.

Screen grid 605g may be connected to B+, suppressor fillsr to cathode 600 within tube 65, and anode 60a through R. F. choke 61 to 3+, also through condensers 53 and 64 in series, resistance 64 and condenser 66 to ground. The common point of condensers 63 and 54 may be connected to the common point of condensers l8 and I9.

Control tube 50 may be provided. This may, and preferably will, be a tube of the thyratron type, having cathode 500, control grid 5&9, and anode 50a. Cathode 500 may be connected to ground through resistance 51!, and the control grid through resistance 53 to switch contact 32. The control grid may likewise be by-passed to ground through condenser 52. The anode may be connected through relay actuating winding to stationary switch contact 46, thence through movable switch contact 41 to B+, and cathode 500 may. be connected through resistance 48 to 3+.

Movable contact 41 may be provided with push button 41a, on depression of which the circuit is opened between contact 41 and contact 46, opening the plate circuit of tube 50. Relay coil 45 of relay 44 controls the armature 45a, which may be spring biased to back contact position to engage contacts 42 and 43. Tuner motor 48 may be energized by a suitable source of potential 4] connected to ground and to one terminal of the tuner motor, the other terminal of which may be connected to contact 42, and, when the relay is closed, to armature 45a, and thence to ground. It will be understood that when relay 44 is closed, control grid 60g of automatic frequency control tube is connected to ground through relay contact 43 and armature 45a, and no voltage is applied to that grid.

It is preferable to tune always in the same direction when the signal-seeking action is to be utilized, and for this reason the motor is not reversible. Return of the cores to starting position may be had by the use of a cam such as shown and described in the co-pending appli-' cation of Alvin D. Meyer and Carl F. Carlzen, Serial No. 560,290, wherein the cores are driven by cam 80, in turn driven through motion reducing means from the motor shaft, and the profile of the cam is so shaped that during a large part of 360 rotation of the cam, the cores move slowly to scan the spectrum, while at or near the 360 position the cores return quickly to initial position, under spring pressure. If this return is fast enough, the tube does not have time to operate during the return travel of the cores.

If condenser tuning is used, it is desirable to arrange the condensers for 360 rotation, and to provide a cam switch in the plate circuit of tube 50 for disabling that tube during the return half of the condenser rotation.

The operation of our receiver will now be described. The frequency discriminator network serves as a source of three voltages. These are respectively: (1) The audio signal to be reproduced (in the case of frequency modulated signals only), (2) a stopping signal to be delivered to motor control tube 50, and (3) an automatic frequency control correcting voltage delivered to tube ((2) and (3) are delivered both for fr quency and amplitude modulated signals). This last lsdesirable in this type of signal-seeking receiverforfrequencymodulatedsignals, since mistuning will likelyoccur if--the modulation, that is, the frequency deviation fromthe mean carrier frequency, is large-at the'time the receiver is tuning into the signal. This is because the point Where the stopping occurs depends upon the instantaneousfrequency of the signal.

If the stopping point 'weredetermined by'the average signal frequencies rather than the instantaneous values, proper tuning would beaccomplished; but ifthe'output ofthe frequency discriminator network ispassed'thro'ugh a low pass filter of proper design; the resulting voltage is the same as for an unmodulated carrier, with the same frequency as 'theaver'age frequency of the modulated signal. I g

This filter cannot usually be-placed in the motor circuit because there is considerable time lag in passingthrough the filter, and thus the tuning would have passed the 'p'ropervalue by the time the signal had passedthroughthe network in the stopping circuit. "However, we may use such a filter network made up" of resistance 55 and condenser 56 to operate frequency control tube 60. This filter is so designed as to remove all'audible frequenciesfrom the signal supplied to tube 60, passing theretoonly sub-audible frequencies.

The action of thisvoltage onthe tube is to shift the oscillator frequency in the direction of proper tuning. While it is unable tocorrect completely for possible mistuning, a correction factor of 5 is readily obtainable. The significance of this is thatform'istuning of as much as 56 kc. the frequency control tube will reduce this mistuning to only kc.

When the receiver is set for amplitude modulation signals, the audio signal is obtained from the output of amplitude modulation detector 3%, which may be a diode which receives its intermediate frequency voltage from the output of intermediate frequency amplifier 2|. The stopping signal and the automatic frequency control voltage to the motor are still furnished by the frequency discriminator network,'but the provision of the automatic frequency control tube reduces the accuracy required of the stopping circuit.

Impulse and hiss type noise will cause very little output in a balanced frequency discriminator network, and this reduction is still further improved by the use of'limiter 22 ahead of the discriminator. The limiter provides a constant input to the discriminator resulting in a uniform output of the discriminator regardless of the signal being tuned.

The output of the balanced discriminator; when the receiver tunes through a signal carrier, rises from zero to a high value of one polarity as the signal is approached thendecreases and goes through zero at exact tuning, and rises to a high value of voltage of the opposite polarity While tuning away from the signal, and finally, after reaching a maximum, drops back to zero again. This discriminator output-is'appne'd to the control grid of thyratron 50, which has sufficient residual bias to prevent ionization until the applied signal is above a predetermined value substantially greater than that arising from noise. The signal resulting from tuning in the carrier issufficient to-operate the thyratron, which in turn operates relay 44, opening the motor circuit and applying the automatic frequency control voltage to frequency control tube 60. The polarity of the output of the frequency discriminator net- 6 work and the direction o fitllhiiig are sci-chosen and related that the voltage resulting in approachingthe signal is of the proper polarity to cause thyratron 50- to operate; i. e.,- to make the grid more positive with respect to cathode.

Additional contacts on relay- 44 may be provided for muting speaker 36 while the-motor circuit is closed, but since this is per se nopart of our invention, the same is not shown or described in detail.

' To summarize briefly the-operation of the receiver, suppose the listener wishes to-listen to amplitude modulated signals; He will setswitch 33 to the amplitude'modulation position and willdepress-push button 41a, opening the thyratron plate circuit between contacts 46 and 41. Relay armature 45a will close to back contact, completing the motor circuit andstarting the tuning motor, and also disabling the automatic frequency control tube by grounding its grid.

The motor will begin to operate, and the tuning will vary until a signal is tuned in. As the signal begins to be tuned in, the voltage output from the frequency discriminator network will rise until it exceeds the noiselevel and thereby overcomes the bias on thyratron 50, causing it to fire. At this time relay'armature dsa moves to front contact position,-opening the motor circuit, and the motor will come to a stop.

A voltage which is a function of the mistuning thereupon is applied to the grid of frequency control tube 6d of a magnitude'and polarity to substantially correct for the mistuning through the well-known automatic frequency control action of this tube, and the signal will beimpressed on loud speaker 36.

Should the listener desire to tune in another station, he will momentarilydepress push button 47a, and the actionwill be repeated, the tuner motor moving the tuner until the next signal in the spectrum is tuned in, whereupon the tuning will stop in the manner already de-- scribed.

If the listener now wishes to receive frequency modulated signals, he will set switch 33 to the frequency modulation position and momentarily depress push button 41a, whereupon the receiver will begin to change its tuning. The stopping action for frequency modulated signals is the same as already described, but in this case the signal voltage is now derived from the output of the frequency discriminator network.

We are aware that the frequency bands in which amplitude modulated signals are received are different from those in which frequency modulated signals are received, and that accordingly, in addition to switch 33, wave band switches are usually provided for selectingthe proper values of L and/or C in the tuned circuits associated with tube 15; but'since such arrangements are well known to those skilled in the art and form per se no part of our invention, they are not shown or described in detail.

Such switches may be gangedwith switch 33 so that operating this switch to change from amplitude modulated to frequency modulated signals simultaneously makes the required changes in the tuning circuits.

In the specification we have explained the principles of our invention and the best mode in which we have contemplated applying those principles, so as to distinguish our-"invention from other inventions; and we have particularly a pointed out and distinctly claimed the part, im-

a age/2v 7 provement, or combinationwhich we claim as our invention or discovery.

While we have shown and described certain preferred embodiments of our invention, it will be understood that modifications and changes may be made without departing from the spirit and scope thereof, as will be clear to those skilled in the art.

We claim:

1. A signal-seeking receiver for both amplitude and frequency modulated signals, comprising, in combination, means for picking up radiated signals, a frequency converter, an intermediate frequency amplifier supplied by said converter, an amplitude modulation detector supplied by the output of said intermediate frequency amplifier, a limiter supplied by the output of said intermediate frequency amplifier, a frequency discriminator network supplied by the output of said limiter, electric power means for varying the tuning of said converter, an automatic frequency control tube supplied by the output of said frequency discriminator network, and a motor control tube supplied by the output of said frequency discriminator network.

2. The combination claimed in claim 1, with a low pass filter connected between said frequency discriminator network and said frequency control tube.

3. The combination claimed in claim 1, in which said low pass filter passes only subaudible frequencies to said frequency control tube.

4. The combination claimed in claim 1, with an audio frequency amplifier, a signal indicator, and means for selectively connecting said audio frequency amplifier to the output of either said amplitude modulation detector or said frequency discriminator network. 7

5. The combination claimed in claim 1, in which said frequency discriminator network delivers audio frequency voltages from frequency modulation signals, a tuner correction voltage proportional in magnitude to the degree of detuning and of a'polarity to correct for the same from either amplitude or frequency modulated signals, and a biasing voltage from either amplitude or frequency modulated signals to permit the motor control tube to pass current in response to an incoming signal of predetermined strength.

6. A signal-seeking receiver for both amplitude and frequency modulated signals comprising, in combination, means for heterodyning incoming signals and for amplifying the heterodyned signals, a detector for amplitude modulated signals, supplied by said amplifying means, a limiter supplied by said amplifying means, a frequency modulation detector supplied by said limiter and delivering audio frequency signals from frequency modulated signals only, a frequency correction voltage proportional in magnitude and corresponding in polarity to the degree and direction of mistuning, and a motor control voltage, both the latter derived from incoming signals either amplitude or frequency modulated; a tuner, a motor for driving said tuner, a motor control tube for controlling said motor, and a frequency control tube for correcting mistuning, means for applying said frequency correction Voltage to said frequency control tube, and means for applying said motor control voltage to said motor control tube.

7. Radio receiving apparatus, comprising,- in combination, a tuned circuit containing a variable tuning instrumentality, electric power means for driving said instrumentality, a motor control tube for discontinuing variation of tuning in response to the voltage developed from received signals, a frequency control tube for applying a correction for mistuning independently of the operation of said tuning instrumentality by varying the tuning of said tuned circuit while said tuning instrumentality is stationary, a frequency discriminator network supplied by incoming signals, a connection from the output of said discriminator to said frequency control tube, said connection including a low pass filter, and a connection from the output of said frequency discriminator network to said motor control tube.

8. Radio receiving apparatus, comprising, in combination, a tuned circuit containing a variable tuning instrumentality, electric power means for driving said instrumentality, a motor control tube for discontinuing variation of tuning in response to the voltage developed from received signals, a frequency control tube for applying a correction for mistuning independently of the operation of said tuning instrumentality by varying the tuning of said tuned circuit while said tuning instrumentality is stationary, a frequency discriminator network supplied by incoming signals, a connection from the output 0f said discriminator to said frequency control tube, said connection including a low pass filter, and a connection from the output of said frequency discriminator network to said motor control tube, the polarity of said connection being so chosen that the grid of said tube becomes more positive with respect to its cathode as a signal is approached.

9. Radio receiving apparatus, comprising, in combination, a tuned circuit containing a variable tuning instrumentality, electric power means for driving said instrumentality, a motor control tube for discontinuing variation of tuning in response to the voltage developed from received signals, a frequency control tube for applying a correction for mistuning independently of the operation of said tuning instrumentality by varying the tuning of said tuned circuit while said tuning instrumentality is stationary, a frequency discriminator network supplied by incoming signals, a connection from the output of said discriminator to said frequency control tube, said connection including a low pass filter, and a connection from the output of said frequency discriminator network to said motor control tube, said motor being arranged for rotation always in the same direction, and means for preventing the operation of said motor control tube during the return portion of the tuning cycle.

MADISON G. NICHOLSON, JR. JOHN C. PONTIUS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,189,282 Foster Feb. 6, 1940 2,369,542 Dietrich Feb, 13, 1945 

